Prior art forced air/boiler furnaces used in residential and commercial buildings or for enclosed portions thereof are relatively large in size, have poor emission levels, have low thermal efficiency, often require large exhaust systems such as a chimney and are therefore impractical for a number of applications.
The low thermal efficiency of prior art furnaces based on the usable fuel gas, oil or any other combustible material is well documented. Most prior art furnaces have efficiency levels of less than 75% and require large exhaust systems such as a chimney to remove the undesirable products of combustion to the outside atmosphere. Chimneys often exit the products of combustion at temperatures well above 300.degree. F. The more recent "High Efficiency" designs of furnaces have addressed this issue to the extent practical by utilizing existing technology. These high efficiency units employ a draw fan motor to extract the products of combustion from a larger surface area heat exchanger and thereby do not require a chimney. In place of the chimney, the high efficiency furnaces have an exhaust pipe of between 2" and 6" in diameter to dispose of the toxic products of combustion to the outside atmosphere. These newer designed high efficiency furnaces have thermal efficiency of up to 90% but they do not address all of the thermal efficiency issues.
When addressing overall thermal efficiency one issue which needs to be addressed is the source of combustion air. Prior art furnaces draw the combustion air from inside the building or dwelling being heated. This requires a greater volume of air to be heated and thereby reduces overall thermal efficiency. The newer high efficiency designs have addressed this issue by drawing outside air via a tube/pipe for the combustion process.
One of the last issues within the efficiency area which needs to be addressed is the number of on/off cycles. In the Northern areas of the United States and Canada, there can be up to 20,000 such on/off cycles per year. Each on/off cycle creates an unsteady state inefficiency at the start of the cycle which continues until steady state conditions are achieved. A heating unit which could fire over a wider firing range would significantly reduce the number of such on/off cycles and thereby improve the overall thermal efficiency. Likewise, the number of on/off cycles can be directly related to the durability of the unit. By reducing the number of on/off cycles per year, the life expectancy of the heating unit can be significantly increased.
The invention presented here addresses the above efficiency issues by utilizing a unique premix design in combination with a pressurized/high velocity combustion process to provide a variable firing rate which utilizes outside air for the combustion process and requires no exhaust draw fan motor. This furnace displays thermal efficiency levels of 95% and greater and requires an exhaust pipe of less than 1" in diameter. The multi-firing rates and unique process control system within the present invention enables the unit to maximize overall thermal efficiency.
The second area addressed by the present invention deals with the quality of the emissions from the combustion process. Prior art furnaces do not address emission levels and the new "High Efficiency" designs focus on improving efficiency levels with little or no concern for emission levels. Some of the most recent designs of prior art furnaces have attempted to reduce emissions by using a small amount of excess air to reduce the flame temperature and thereby reduce the NOx emission levels. The present invention addresses the quality of emissions issue through its uniquely designed premix system which results in a more efficient mixing of the reactants and thus results in the combustion process going to completion. The high velocity/super atmospheric premixture also results in increasing the rate of combustion and thereby significantly lowering the formation of NOx. The present invention also uses excess air within the combustion process to further improve the emission levels.
The third area addressed by the present invention is the physical size and weight of the heating unit. Prior art furnaces of both the conventional and the high efficiency design utilize a relatively large amount of space and are excessively heavy and bulky. The size/weight issue with the prior art furnaces makes certain applications impractical when combined with the flue exhaust requirements of a chimney or larger diameter pipe. These flue exhaust requirements make it impractical in many apartment and commercial buildings to use the high efficiency furnaces. As a result, these apartment and commercial buildings must go to the costlier less thermal efficient electric heaters. The present invention utilizes a very high convective heat transfer process which makes the unit considerably smaller and more compact. The furnace in the present invention is not only compact but it is insensitive to its placement enabling the same unit be placed in a horizontal or vertical position. The present invention requires an exhaust tube/pipe of less than 1" in diameter which can be of a flexible design to accommodate a variety of routing configurations. The high efficiency gas furnace of the present invention being light weight, compact and requiring a small exhaust tube/pipe enable it to be installed inside closets, cupboards or similar type spaces.